Image:
Talking about this header, and this button. Which are located in the Administration panel.
As you can see, the button is pressed but does not generate the area to put text/code in.
Same goes for the custom css.
This is the core we are talking about, and not an added extension.
What I did before this occured was add a <script> code for a popunder ad,
and after doing so, it no longer worked.